1. Field of the Invention
The present invention relates to electrophotographic image-forming apparatuses such as laser beam printers or LED printers.
Due to recent developments in office automation, electrophotographic image-forming apparatuses, such as laser beam printers, have been widely used in computer output terminals, copying machines, facsimile machines and others.
In such image-forming apparatuses, a hard copy is obtained by the steps of charging a photosensitive drum to a predetermined potential by a charger, forming an electro-static latent image corresponding to an image information on the photosensitive drum by irradiating a light beam thereon, developing the latent image with a toner, transferring the developed image to a recording medium, and fixing the same.
After the transfer step, the photosensitive drum is discharged by a discharger, and the residual toner is scraped off from the surface of the photosensitive drum by a cleaner, thus one cycle of the printing operation is completed on the photosensitive drum.
2. Related Art
A so-called "one component and a half" development has been known for developing the electrostatic latent image on the photosensitive drum, wherein a magnetic carrier is stored in a carrier chamber or a development chamber which is then replenished with a toner.
This method provides a development device simple in structure because no control is necessary for a toner density. However, to maintain a favorable image quality, means is necessary for maintaining the toner density at a constant value.
Also a structure has been known for recycling the residual toner scraped off from the photosensitive drum by the cleaner and returning the same to a toner box through a toner recycling mechanism for the purpose of reuse. A development device with such a recycling mechanism is advantageous because the residual toner is not discharged as waste whereby the efficiency for reusing the toner is improved.
According to the conventional toner recycling mechanism, however, since the residual toner must be returned to the toner box mounted above the development device, it is necessary to convey the residual toner obliquely upward. Thereby, there is a problem in that the residual toner is liable to dwell on the bottom of the cleaner housing, resulting in the lowering of the toner reuse efficiency.
In addition, if foreign matter such as paper chips or dust are mixed in the residual toner, they may be conveyed to a developer roller and nipped between a doctor blade and the developer roller. As a result, there is a problem in that the conveyance of the developing agent is impeded at a location where the foreign matters are nipped, whereby an incomplete printing may occur at a portion corresponding to this location.
In a device for carrying out the "one component and a half" development, a carrier chamber for storing a toner is defined between a developer roller and a partitioning member. When the developer roller rotates, the toner is supplied from a toner chamber to the carrier chamber in which the toner is mixed with the carrier and charged at a predetermined potential level.
However, in the conventional carrier chamber, since a distance between the partitioning member and the developer roller becomes gradually smaller near the ends thereof, there is a tendency that the developing agent moves from the end zones to the central zone whereby the amount of carrier is reduced in the end zones.
In such a case, the toner density becomes higher in the end zones because the toner supply from the toner chamber is uniform along the axis of the developer roller, resulting in the unevenness in the toner density. The unevenness in the toner density causes the deterioration of qualities of the printed image, such as shade irregularity or fog.